The invention relates to imaging systems. In particular, the invention relates to imaging systems using point source illumination.
Imaging systems are used in a wide variety of important and often critical applications, including but not limited to, identification and inspection of objects of manufacture. In general, imaging systems either employ ambient illumination or provide an illumination source to illuminate an object being imaged. In the most familiar imaging systems, the illumination source is based on some form of electromagnetic radiation, such as microwaves, infrared, visible light, Ultraviolet light, or X-rays. Aside from various forms of electromagnetic radiation, the other most commonly employed illumination source found in modern imaging systems is one that is based on acoustic vibrations as such as is found in ultrasound imaging systems and Sonar systems.
The imaging system forms an image or picture of an object by measuring and recording a response from the object to the illumination produced by the source. In addition to utilizing a variety of illumination sources, imaging systems known in the art employ a variety of illumination source geometries to facilitate imaging. The illumination source geometry defines the relative location and orientation of the illumination source with respect to the object. Among the common illumination geometries used in imaging systems that provide an illumination source are collimated illumination, coaxial illumination and point source or isotropic illumination. Point source illumination is illumination using an isotropic source located near to the object and therefore, also sometimes is referred to as isotropic illumination.
An example of the use of point source geometry in conjunction with X-ray illumination can be found in X-ray laminography. X-ray laminography is an imaging technique that facilitates the inspection of features at various depths within an object. Usually, an X-ray laminography imaging system combines multiple images taken of the object to produce a single image. The multiple images are often produced by moving an X-ray point source around the object and taking or recording the images using different point source locations. By taking images when the source is at various locations during the movement of the source, the combined image is able to depict characteristics of the internal structure of the object. In some instances, such as in analog laminography, the images are combined directly during the process of taking the images. In digital X-ray laminography, the individual images are combined digitally to produce a combined image. An application of X-ray laminography is the inspection of multilayer printed circuit boards (PCBs) and integrated circuits used in electronic devices.
The illumination of an object by a point source of illumination located above but relatively close to the object, such as an X-ray source used in conventional X-ray laminography, does not produce uniform illumination of the object. Instead, the illumination tends to vary across the surface of a plane upon which the object is resting as a function of distance from the point source. The illumination variation or unevenness is due to the point source acting as an isotropic radiator. An isotropic radiator exhibits a decrease or diminution in radiation or illumination intensity that is entirely a function of distance, as opposed to a function of distance and direction. The non-uniform illumination produced by the point source on a planar surface results in an apparent variation of brightness in the image produced. In traditional analog X-ray laminography, the brightness variations are generally not a problem because the relative motion between the object and X-ray source during the imaging process are configured such that when the images are combined, the effects of the non-uniform illumination are averaged out.
On the other hand, the brightness variation or brightness unevenness due to point source illumination can and does interfere with imaging applications that do not combine the images as in traditional laminography. In general, brightness variations across an illuminated object in an imaging system are used as the basis for constructing the image. Therefore, brightness variations associated with the characteristics of point source illumination can and do interfere with the quality of the image, since these variations are not a function of the object being imaged, but instead are a function of the illumination geometry. For example, 3D digital tomography can utilize a series of images taken from various illumination angles to reconstruct a 3-dimensional representation of the object being imaged. Point source illumination effects tend to interfere with the reconstruction of the 3-dimensional imaging created by 3D tomography due to the aforementioned unevenness of the illumination.
Accordingly, it would be advantageous to have a method and apparatus for brightness correction that could minimize or largely eliminate the effects of using a point source to illuminate an object. In addition, it would be desirable if the method and apparatus did not require precise knowledge of the relative locations of the point source and the object to accomplish brightness correction. Such a method and apparatus would solve a long-standing need in the area of imaging systems using point source illumination geometries.
The present invention is a novel method and apparatus of brightness correction or brightness compensation for use in imaging systems employing a point source of illumination. The brightness correction method and apparatus of the present invention perform correction by compensating for the difference in distance from the point source to various points on the surface of a plane or to the planar-like surface of the object itself. Generally, the method and apparatus of brightness correction utilize knowledge of the location of the source relative to the planar surface. However, advantageously the method and apparatus may be applied to correct for brightness variation in the absence of accurate and precise knowledge of the location of the point source. The method and apparatus are applicable to imaging and inspection systems used with a wide variety of planar and semi-planar objects including but not limited to X-ray inspection of printed circuit boards (PCBs) and integrated circuits.
In one aspect of the invention, a method of brightness correction of an image is provided. The method comprises a step of measuring one or more brightness values for each pixel in the image. The method further comprises a step of computing a corrected brightness for each pixel using a closed form equation. The method uses knowledge of the location of an illumination source.
In yet another aspect of the invention, a method of brightness correction of an image is provided that does not require knowledge of the location of the source of illumination. The method comprises a step of measuring one or more brightness values for each pixel in the image. The method further comprises a step of minimizing a function that represents a sum of the square of a difference between pairs of corrected brightness values for symmetrically located pixels. The result of the step of minimizing is a set of corrected brightness values and a set of geometry parameters that specify the location of the source.
In yet another aspect of the invention, an apparatus for brightness correction is provided. The apparatus of the present invention is a computer having a memory. A computer program is stored in the computer memory. The computer program implements the method of the present invention.
In still yet another aspect of the invention, an imaging system for imaging planar and semi planar objects is provided. The imaging system comprises point source illumination of the object and the apparatus of the present invention described above, which implements one or both of the methods of the invention described above. The imaging system images a wide variety of planar and semi-planar objects, including but not limited to X-ray inspection of printed circuit boards (PCBs) and integrated circuits.